Refactor build routine and switch to airbnb style (#1703)

- Relocate webpack stuff to assets/build
- Switch all js to airbnb code style
- Update webpack to v2.0 (beta)
- Update all other dependencies
- Assets manifest uses relative path as key [1]
- Code should be compatible with Node 4.5.0
- All dependencies have been updated

[1]: Previously the assets manifest would use the basename of a file path as
its key when looking up a cachebusted file. This was to be consistent with
Sage 8. This change makes it so that the relative path is used instead.

To clarify, review the following example.

Before:
```
{
  "main.js": "scripts/main_5f4bfc9a9f82291c6dea.js",
  "main.css": "styles/main_5f4bfc9a9f82291c6dea.css",
  "customizer.js": "scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

After:
```
{
  "scripts/main.js":"scripts/main_5f4bfc9a9f82291c6dea.js",
  "styles/main.css":"styles/main_5f4bfc9a9f82291c6dea.css",
  "scripts/customizer.js":"scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

assets/build/webpack.config.production.js

@@ -0,0 +1,40 @@
+/* eslint-disable import/no-extraneous-dependencies */
+const AssetsPlugin = require('assets-webpack-plugin');
+const OptimizeCssAssetsPlugin = require('optimize-css-assets-webpack-plugin');
+const cssnano = require('cssnano');
+const path = require('path');
+
+const config = require('./config');
+
+/**
+ * Process AssetsPlugin output and format it
+ * for Sage: {"[name].[ext]":"[name]_[hash].[ext]"}
+ * @param  {Object} assets passed by processOutput
+ * @return {String}        JSON
+ */
+const assetsPluginProcessOutput = (assets) => {
+  const results = {};
+  Object.keys(assets).forEach(name => {
+    Object.keys(assets[name]).forEach(ext => {
+      const filename = `${path.dirname(assets[name][ext])}/${path.basename(`${name}.${ext}`)}`;
+      results[filename] = assets[name][ext];
+    });
+  });
+  return JSON.stringify(results);
+};
+
+module.exports = {
+  plugins: [
+    new AssetsPlugin({
+      path: config.paths.dist,
+      filename: 'assets.json',
+      fullPath: false,
+      processOutput: assetsPluginProcessOutput,
+    }),
+    new OptimizeCssAssetsPlugin({
+      cssProcessor: cssnano,
+      cssProcessorOptions: { discardComments: { removeAll: true } },
+      canPrint: true,
+    }),
+  ],
+};